Conditions for defect-free solidification of aqueous ammonium chloride in a quasi two-dimensional directional solidification facility

Experimental and theoretical approaches to the problem of the solidification of a binary melt are presented. We describe a series of experiments in which aqueous solutions of ammonium chloride of above eutectic composition are cooled from below in a Hele-Shaw cell that is translated downwards at a constant rate. A mushy layer of solid dendrites bathed in a salt-depleted interstitial fluid is formed. For certain combinations of initial concentration and translation speed, convection in the mush gives rise to chimneys which result in defects in the solidified material. Our experiments demonstrate that using a higher temperature gradient for solidification increases the range of conditions for which a mushy layer forms that is free from both chimneys and secondary nucleation. We present a mathematical model of the system, to which we derive an approximate analytical solution. Predictions of the height of the mushy layer given by this approximate solution are found to be in excellent agreement with full numerical predictions when the height is small. Both numerical and analytical predictions are a better match to experimental data when the temperature gradient is low.